Several types of optical communication devices comprise optical waveguides, optical modulators, and optical switching structures made of electro-optic material. A substrate optical waveguide comprises a lower cladding layer formed on the substrate, a core layer having a higher index of refraction formed over the lower cladding layer, and usually an upper cladding layer formed over the core layer. An optical modulator, or an optical switching structure, may be formed in line with the optical waveguide by forming a body of electro-optic (E/O) material on the same level as the core material, with the electro-optic material usually sandwiched between upper and lower cladding layers. Two electrodes are formed on opposing surfaces of the body E/O material, and are used to apply an electric field to the E/O body. The electric field changes selected optical properties (e.g., refractive index, polarization) of the E/O material. The changes in optical properties may be used to achieve various types of modulating, switching, and filtering functions.
A coefficient may be used to relate the change in the optical property of the material with respect to the applied electric field (i.e., the applied voltage divided by the dimension of the material along which the voltage acts). Electro-optic materials are usually crystalline materials or highly ordered materials (as in the case of polymers). In both cases, the value of the electro-optic coefficient usually depends upon the direction of the electric field relative to the orientation of the material's crystal or highly-ordered structure. Because of this, the electro-optic property is usually specified as a matrix of coefficient values, each of which is measured along a different axis of the material's crystal or ordered structure. This matrix is often called the tensor matrix of the material's property.
In electro-optic devices used in large systems integrated on substrate carriers, the E/O material usually comprises an inorganic single crystalline material, such as lithium niobate, which is difficult to grow and pattern. However, such single crystalline materials have relatively low responses to the applied electric field compared to other inorganic crystalline materials, such as lanthanum-modified lead zirconium titanate (PLZT). But such crystalline materials cannot be easily formed on substrate carriers, and must be grown on top of a base crystalline substrate in order to cause the material to form a crystalline structure. This precludes using the crystalline materials in small-scale modulators and switching devices that have vertically oriented electrodes (top and bottom electrodes) since the material cannot be grown over the bottom electrode. Thus, the potential of using inorganic crystalline materials in these devices cannot be realized. The inventors have thought of, and considered, an approach of making large-sized optical modulators and switches that comprises the steps of: (1) forming 100 μm thin wafers of inorganic E/O material, (2) followed by coating top and bottom surfaces with metal, (3) dicing the substrate into small pieces, and (4) then bonding the bottom surfaces of the pieces to metal pads on the substrate carrier, which will hold all of the electro-optic devices. However, this approach is not currently able to construct small-sized electro-optic switches and modulators (less than 10 μm in thickness and width), and is not practical for large-scale integration of electro-optic devices. This approach is a conception of the inventors, and does not form a part of the prior art to the inventors' knowledge.
FIG. 1 shows a part 5 of a prior art electro-optic device that uses inorganic crystalline material or inorganic poly-crystalline material 6 and two electrodes 7 and 8. Part 5 may be incorporated into an interferometer-type optical switch or a polarization-type modulator. Inorganic E/O material 6 is grown as a layer over a dielectric crystalline substrate 1. The crystal lattice constant of E/O material layer 6 and substrate 1 are closely matched, and material 6 is grown with a crystal orientation that is set by the crystal orientation of substrate 1. After E/O material layer 6 is initially formed, it is pattern-etched to form a mesa ridge 9 between the locations where electrodes 7 and 8 are to be formed. Mesa ridge 9 has a width W, a length L, and a height h. In use, a light beam will be conveyed through mesa 9 along the length L. E/O material 6 of mesa 9 has a refractive index that is higher than that of substrate 1, and higher than that of the air above mesa 9 (in this case, the air effectively acts as an upper cladding layer). The higher refractive index provides vertical confinement of the light beam within mesa 9. The step difference in height h causes the refractive index of the material 6 underneath mesa 9 to be higher than that of material 6 that is to the side of mesa 9, which provides lateral confinement of the light beam within mesa 9. After mesa 9 is formed, electrodes 7 and 8 are formed, and are separated by a distance SE. The electrodes generate an electric field along distance SE, which can then be used to modify the optical properties (e.g., refractive index, polarization) of E/O material 6 within the lower part of mesa 9. Unfortunately, the direction of this electric field is usually oriented along one of the smaller coefficients of the material's tensor matrix, which requires one to use a higher voltage between electrodes 7 and 8 to achieve a desired change in optical properties. In addition, because the spacing distance SE is usually on the order of 6 μm to 10 μm, and because the electrodes are oriented parallel to the lines of the electric field, the effective separation distance of the electrodes for generating the electric field is on the order of 8 μm to 12 μm. This large effective distance also increases the amount of voltage needed. The requirement for higher voltage reduces the modulation rate or switching rate of the electro-optic device that incorporates part 5.
Accordingly, the inventors have recognized that electro-optic devices with higher modulation/switching rates could be made if the above problems were solved.